The top face of a piston of a direct fuel-injection diesel engine is generally formed as a flat face, but a direct fuel-injection diesel engine having a piston with a top face that projects in a pentroof shape is known from Patent Document 1 below.
When a cavity is recessed in the top face of a pentroof-shaped piston, the height of an opening of the cavity varies in the circumferential direction. Therefore, if the height of a bottom wall portion of the cavity is made uniform in the circumferential direction, the depth of a peripheral wall portion of the cavity varies in the circumferential direction, and the conditions in which air and fuel injected from a fuel injector are mixed become nonuniform in the circumferential direction, thus giving rise to the problems that the engine output decreases and harmful exhaust substances increase.
In order to solve such problems, in the arrangement described in Patent Document 1, the height of the bottom wall portion of the cavity is varied so as to follow the variation in height of the opening of the cavity, thus making the depth of the peripheral wall portion of the cavity substantially uniform in the circumferential direction and thereby making the conditions in which fuel and air are mixed in the cavity uniform in the circumferential direction.    Patent Document 1: Japanese Patent Publication No. 5-17369